Nuclear-fuel element

ABSTRACT

A fuel element for a nuclear reactor comprises a hollow structure forming a coolant path and one or more partitions intercepting the coolant flow between the inlet and the outlet, the partitions comprising perforated, aperture or porous supporting walls flanking a porous mass of coated nuclear-fuel particles.

United States Patent Barthels et al. Apr. 2, 1974 NUCLEAR-FUEL ELEMENT3,179,572 4/1965 Perilhou et a1. 176/73 [75] Inventors: Heinz Barthels;.llosei Fasshentier; A87 6 Cannon 176/73 We Katscher, all of Julich,FOREIGN PATENTS OR APPLICATIONS Gemlany 845,804 8/1960 Great Britain176/73 [73] Assigneez Kernmrschumgwnuage lunch 946,901 l/1964 GreatBritain... 176/73 Gesenschafi mat beschlmnmer 998,387 7/1965 GreatBrItam 176/73 Hammg Julich Germany 1,464,740 9/1969 Germany 176/73 [22]Filed: 1971 Primary ExaminerReuben Epstein 21 App], 299,301 Attorney,Agent, or Firm-Karl F. Ross; Herbert Dubno [30] Foreign ApplicationPriority Data Dec. 24, 1970 Germany 2063876 [57] ABSTRACT 52 115.131176/73 176/82 176/83 A fuel element 8 nuclear reactor comprises a 511111. c1 @216 3/16 10w smmure forming a coolant Path and one or more[58] 1 16111 61 Search 176/73 82 83 Panitkms intercepting c0013 betweeninlet and the outlet, the partitions comprising perfo- [56] ReferencesCited rated, aperture or porous supporting walls flanking a porous massof coated nuclear-fuel particles.

11 Claims, 13 Drawing Figures PATENTEDAPR 2 mm 3.801.450 sum 1 av 4hiijw MENIEM 2 9 3,801,450 SHEU 2 U? 4 wgmgmra 21914 3,801; 450

sum 1 or 4 D FIG. 9

COAT/N6 NUCLEAE FUEL FIG.

NUCLEAR-FUEL ELEMENT FIELD OF THE INVENTION Our present inventionrelates to a fuel element for a nuclear reactor and, more particularly,to a nuclearfuel element for a reactor core using coated-particle fuels.

BACKGROUND OF THE INVENTION Coated-particle nuclear fuels generallyconsist of a core of a fissionable material, e.g. plutonium, uranium orthorium oxides or other compounds, individually or in mixtures, encasedin a ceramic-like sheath or coating designed to retain gaseous and solidfission products, to

' protect the fuel core and to allow heat exchange with the reactorcoolant. The coated-particle fuel generally has a diameter between and3X10 microns, the coating being pyrolytically deposited carbon, siliconcarbide, or like refractory, high-strength materials. Coated particlesintended for use with the present invention may be any of thosedescribed in the article entitled Ceramic Coated-Parzicle Nuclear Fuelsby Dayton, Oxley and Townley, JOURNAL OF NUCLEAR MA- TERIALS, Volume 11,No.1, pages 1 31 (January 1964).

It has been proposed to use such coated-particle nuclear fuels directlyin high-temperature gas-cooled nuclear reactors, to incorporatecoated-particle nuclear fuels in metal casings or graphite bodies and touse coated particles in various other nuclear-fuel elements ofconventional configuration. The fuel elements were elongated orspherical, tightly packed or highly porous and generally wereconstructed to allow the coolant to pass through the interior of thefuel element or through a packing of such fuel elements. When metalcasings were employed, they generally were hollow to form a coolantchannel through the center thereof.

While nuclear-fuel elements of the aforedescribed character weresatisfactory for most purposes, they were only limitedly suitable foruse in research reactors in which the presence of large radiationchannels was desirable. Moreover, efficient heat exchange with particlesof nuclear fuel in the interior of the fuel element could not always beguaranteed. Furthermore, it was often desirable to control thedistribution of the coolant with respect to the character of the nuclearfuel to obtain a predetermined relationship, this being difficult, ifnot impossible, with bulk-fuel elements of the character described.

To overcome these disadvantages, it was proposed to provide thecoated-particle nuclear fuel as a porous body with the coated particlesbonded together or agglomerated to form a porous body. The gas or otherreactor coolant could then be passed directly through this body, Thissystem had the disadvantage that it was difficult to control gasdistribution in relationship to the characteristics of the fuelparticles and the further drawback that the body had insufficientmechanical stability and resistance to abrasion.

To improve the uniformity of interaction ofa gaseous coolant and anuclear fuel, it has been suggested to provide a nuclear fuel body as alayer of a thickness between 3 and 40 mm and to deflect the coolantstream through this layer by closing the central passage ofa cylindricalor annular structure constituted of the nuclear-fuel material. Thethickness of the layer was limited because of the high pressure drop andflow resistance imposed by the nuclear-fuel layer. Mechanical breakdownand rupture of the nuclear-fuel layer was, however, a significant dangersince it tended to shut the coolant around the nuclear fuel and therebyallow overheating in regions of the reactor. Other arrange ments usingporous layers of nuclear-fuel particles have been characterized by thenonuniformity of distribution of the coolant, inefficient heat exchangebetween all of the nuclear fuel and the coolant, excessive pressure dropand fluid-flow resistance, the danger of coolant breakthrough andshunting, and difficulties with respect to the mechanical strength andability to handle fuel elements containing the nuclear-fuel particles.It should also be noted that an important consideration in dealing withnuclear-fuel arrangements is the disability of a high spatial density ofthe nuclear fuel while permitting, at least in research reactors,radiation channels to exist into which a specimen to be radiated can beintroduced.

OBJECTS OF THE INVENTION It is the principal object of the presentinvention to provide an improved nuclear-fuel element which has a highyield, which affords uniform cooling, and coolant flow and which avoidsthe difficulties hitherto encountered with prior-art nuclear-fuelelements.

Another object of the invention is to provide a nuclear-fuel elementwith uniform and controllable distribution of a cooling fluid withoutthe danger of coolant breakthrough or over-heating of the nuclear fuel.

It is also an object of our invention to provide a nuclear-fuel elementparticularly adapted for use in a research reactor and which avoidsdifficulties hitherto encountered with particulate nuclear fuels.

Still another object of the instant invention is the provision of animproved nuclear-fuel element with a long useful life.

SUMMARY OF THE INVENTION These objects and others which will becomeapparent hereinafter according to the present invention, are obtainedwith a nuclear-fuel element for use in a nuclear reactor, e. g., ahigh-temperature gas-cooled nuclear reactor which comprises a housing,shell or casing defining a concurrent-flow passage having an inlet sidefor the coolant, and an outlet side, the passage being spanned by one ormore partitions, each of which comprises a pair of fluid-permeable wallsin spaced-apart relation and receiving a filling of coated particles ofnuclear fuel. The term nuclear-fuel element as used herein, is thusintended to describe the structure containing the passage and one ormore such partitions which is used to contain the nuclear fuel and isinsertable into the nuclear-reactor core in a predetermined matrix orarray so as to generate a neutron flux or to be irradiated thereby.

The arrangement of a loosely piled mass of coated nuclear-fuel particleshas been found to have some significant advantages in the operation ofthe reactor. Firstly, the perforated walls flanking the mass may have aflow-passage cross-section controlled either by varying the number oforifices, or the cross-section of each orifice, or the thickness of thewall traversed by each orifice to control the throughflow resistance.Each of these parameters may, of course, be used in conjunction with anyother parameters. The flow cross-section and resistance may thus bedependent on the packing of the fuel particles or may be used inconjunction with the packing to obtain the desired distribution ofcoolant vis-a-vis the nuclear fuel or to maintain a uniform flow throughall portions of the partition.

According to an important feature of the invention, the supporting wallsof each partition, flanking the mass of loosely piled coatednuclear-fuel particles, can comprise perforated plates, sieve members orwire fabric or mesh, or even plates of coherent nuclear-fuel particlesof a predetermined porosity. In all of these cases, it is possiblesimply by adjusting the pore size, number of pores or distribution, orthe thickness of the wall through which the coolant must pass, torestrict or increase the flow resistance to maintain the coolant flowproportional to the neutron flux of the fuel material. In other words,in regions in which the neutron flux of fission reactions is greater,the coolant flow may be increased and vice versa. When the walls arecomposed of coated particles or nuclear fuel bonded together, theporosity may be regulated by increasing the proportion of binder incertain areas to reduce the pore crosssection and vice versa.

According to a more specific feature of the invention, the inlet andoutlet passages formed in the walls of each partition turned toward theinlet and outlet sides of the coolant duct, respectively, are designedto maintain the coolant velocity in the axial direction substantiallyconstant. In this case, the duct may provide an axial inlet and an axialoutlet for the coolant as well as inlet and outlet duct branches whichparallel one another in the axial direction over at least a portion ofthe length of the fuel element but are separated by one or morepartitions. Since the rate at which the coolant traverses the partitionis a function of the flow crosssections of the inlet and outlet passagesof the walls forming the partition, and the pressure differentialthereacross, we prefer to vary the flow direction at an acute angledefined by the half-angle of the conicity of the partition. Thefrustocone may converge or diverge in the flow direction and we preferto provide a multiplicity of frustocones in alternately oppositeconvergence and in axial alignment within the interior of the fuelelement, The apexes of the conical bodies preferably abut one another.While the cones may be defined by straight-line generatrices, we maymake use of conical partitions whose generatrices are arcs, preferablycorresponding to the arc of a sinusoid whereby the partition may havethe configuration of a corrugated pipe.

According to another feature of the invention, the relationship betweenthe thermal (and neutron flux) output and the coolant flow may be variedby adjusting the diameter and/or the nuclear-fuel content of thenuclear-fuel particles within the fuel element.

Furthermore, we have found it to be advantageous, especially when themass of coating nuclear-fuel particles is disposed between more or lessrigid walls to provide one or more layers of yieldable butfluidpermeable material, preferably of a substance having a lowneutron-absorption cross-section, in contact with the body of coatedparticles. The layer may be provided between the body of coatedparticles and either or both permeable walls and one or more layers maybe provided within the body of coated particles. The layer may be a mat,felt or fabric of graphite, silica or like fibers capable of resistingthe high temperatures of the reactor and yielding to the thermallyinduced expansion and contraction of the body of nuclear-fuel particles.This prevents the particles from becoming damaged by movement againstthe walls, limits the formation of hollows within the mass of looselypacked nuclear-fuel particles, and otherwise reduces stress in thesystem.

It should be noted further that the present invention allows thethickness and/or porosity of the loosely piled layer of coatednuclear-fuel particles and/or the thickness and/or the porosity of theporous walls flanking the partition to be varied over the length of thefuel element to achieve uniform distribution of the coolant or acontrolled distribution of the coolant in accordance with variations inreactor output.

Among the advantages of the fuel element according to the presentinvention is that it allows a substantial improvement in the heattransfer between the fuel and the coolant because the coolant in allcases passes transversely through the body of nuclear fuel particles ina distributor manner. In other words, all of the coolant passes throughthe partition at least once. The advantages of the controlleddistribution of the coolant and an optimum adjustment of the coolantflow to the thermal output of the fuel element has also been discussed.Moreover, the fuel element of the present invention provides forincreased safety since the construction of the fuel-particle body as apartition increases the availability of channels for emergency flow ofcoolant. It suffices, therefore, to introduce coolant at the outside ofthe nuclear fuel body to enable it to pass effectively over theremainder thereof. Of course, it is also possible to spray the coolantfrom the inside out, or from the exterior transversely to the surface toobtain a satisfactory coolant and for this reason the fuel elementscontemplated in accordance with the present invention can be usedeffectively in water-cooled nuclear reactors. This arrangement has thefurther advantage that, upon failure of the primary coolant flow, thefuel element remains surrounded by the secondary coolant.

Still another advantage of the fuel element according to the presentinvention resides in the fact that the nuclear-fuel body formed by thepartition can be held to a thickness of one to several millimeters, sothat additional space is provided, aside from the coolant channels,which may be used in a research reactor for central radiation channelsand the like.

DESCRIPTION OF THE DRAWING The above and other objects, features andadvantages of the present invention will become more readily apparentfrom the following description, reference being made to the accompanyingdrawing in which:

FIG. 1 is anaxial cross-sectional view, partly in diagrammatic form, ofan annular nuclear-fuel element according to the invention;

FIGS. 2 and 3 are cross-sectional views of annular fuel elementsaccording to the invention, wherein the parition has a conicalconfiguration, only one side of the axially symmetrical fuel elementbeing shown in each case;

FIG. 4 is a cross-sectional view showing one side of the annular nuclearfuel element provided with a corrugated-tube partition in which thecross-section corresponds generally to a sinusoid;

FIGS. 5 7 are axially cross-sectional views through tubular fuelelements according to the present invention, showing otherconfigurations of the generally conical partition;

FIG. 8 is a cross-sectional view taken generally along the line VIIIVIII of FIG. 1 and drawn to an enlarged scale;

FIG. 9 is a detail view of a portion of the partition of FIG. 1 butillustrating a region further along the fuel element;

FIG. 10 is a cross-sectional view through a nuclear fuel particleaccording to the present invention at one portion of the partition;

FIG. 11 is a cross-sectional view through the nuclear fuel partition atanother portion of the partition; and

FIGS. 12 and 13 are diagrammatic cross-sectional views through otherpartition structures embodying the" present invention.

SPECIFIC DESCRIPTION In FIG. 1, we have shown a nuclear fuel elementwhich comprises an outer cylindrical casing 1 and an inner cylindrcialcasing 2 coaxial therewith. The casing l defines an axial inlet la for acoolant fluid and is perforated at its upper end 1b at 1c to defineoutlet passages for the coolant. To either side of the body of thenuclear fuel element, therefore, there is provided an axial inlet regionId and an axial outlet region 12 which may be traversed by the coolant.The annular duct formed by the body of the fuel element, between thepassages 1d and leis partitioned at P by an annular partition consistingof, in its basic construction, a loose filling 3 of coated-particlenuclear fuel and a pair of perforated walls 4 retaining the filling inplace. Instead of a loose filling of free particles, the particles maybe held in a matrix or hinder, e.g., of silica gel or other refractorymaterial as long as the porosity of the filling is maintained. The walls4 are provided with apertures 4a forming inlet passages, and apertures4b forming outlet passages for the coolant. At its upper end, the tubeformed by the filling 3 and walls 4 is closed by a ring 10 to ensurethat all of the fluid entering the tube traverses the partitiongenerally radially, as represented by the arrows.

Because of the large total throughflow cross-section of the partition,the overall pressure drop is minimized and becuase of the large overallsurface area of the nuclear fuel mass the heat transfer rate isincreased.

Because of the fact that the coated particles have a diameter in generalbelow about 2 mm, the high ratio of heat transfer surface area to volumeprovides a high specific volume of nuclear fuel for a given rate of heattransfer and volume of the fuel element. It is, of course, possible toeliminate the outer casing 1 completely in the event the coolant is tobe discharged into the space surrounding the fuel element. Furthermore,openings may be provided at 1h through which a coolant may be introducedwhen the port la is to constitute an outlet. In this case, the coolantwill traverse the partition in the opposite direction.

Referring now to FIGS. 8 and 9, it will be seen that the outer wall 104and the inner wall 104' may have apertures 104a and 104a ofa certaincross-sectional area and spacing which may differ from the spacing ofthe apertures 104b of the wall portion at another location along thelength of the fuel element. Consequently, the rate of flow of thecoolant across the partition can be adjusted in accordance with the heatgenerated in the particular region of the partition or in accordancewith the pressure drop to main a substantially constant axially flowvelocity throughout the fuel element as illustrated in FIG. 1.Furthermore, the thickness 1 of the rigid wall may be different at onelocation from the thickness T at another location (compare FIGS. 8 and9) so that the flow rate is controlled by the length of the passage 104aor 104b as previously described. Furthermore, the flow rate can becontrolled by the packing density of the coated particles between therigid walls and thus FIG. 9 shows a less dense packing than is shown inFIG. 8.

According to the invention, moreover, layers 11], 112 and 113 of arefractory fluid-permeable material, e.g., graphite fibers in felt orfabric form, may be provided between the layer of coated particles andthe re' spective walls or within the layer of coated particles to takeup variations in dimensions resulting from temperature changes. In allcases, the cushion lyaers should be composed of a material having a lowabsorption cross section for a neutron flux. The thickness D of theparticle layer may also differ from the thickness d at another locationalong the length of the fuel element in order to coordinate the coolantflow rate to the output of the fusionable fuel.

In FIG. 12, we have shown an arrangement in which the outer walls of thepartition are formed by a layer 204 of coated particles 214 heldtogether by a binder 215, the distribution of pores 216 and the meanpore size being controlled by the proportion of binder mixed with theparticles. As in the embodiment of FIGS. 8 and 9, the walls 204 containa loose or bound pile of coated particles 203. In all of the embodimentsdescribed, the proportion of nuclear fuel which is present may beincreased by increasing the diameter of the core 203 which is composedof uranium, plutonium or thorium compounds. The ceramic coating isrepresented at 203" in FIG. 10 and in FIG. 11, we show a similar coatedparticle in which the core 303' has a smaller diameter but is surroundedby a thicker coating 303".

In FIGS. 2 4, we show various sytems embodying the invention and inwhich the axial flow of the fluid into and out of the element isrepresented by the arrows. In FIGS. 2 4, the inner shell is representedat 402, 402a and 402b while the outer shell is constituted at 401, 401aand 401b, the outer shell being formed with inlets and outlets asdescribed in connection with FIG. 1. Between the shells and in theangular gap defined thereby, there is provided the fuel partitions 403,403a and 40312, respectively constituted by layers of coated particlesreceived between pairs of apertured walls 404, 404a and 40422. Thepartition 403, 404 of FIG. 2 is simply a frustocone diverging in thedirection of the outlet and positioned so that the passage through theplates 404 open perpendicularly to the layer 403 and are offset from oneanother to cause the coolant to meander through the fuel layer.

In the system of FIG. 3, two frustocones are provided at C and C' withtheir broad bases in contact so that in approximately the same axialdistance in which the gases in FIG. 2 traverse one partition, thecoolant of FIG. 3 must traverse two partitions. In the system of FIG. 4,the partition 403b, 404b is constituted by a corrugated tube formed bythe walls 404b and packed at 40312 with the coated-particle nuclearfuel. The troughs of the corrugated tube hug the inner casing 40212while the crests are, in turn, tangent to the outer casing 40lb.

FIGS. 7 show embodiments of the present invention wherein a singletubular casing 501, 501a or 50122 is provided with an inlet passage atthe lower end and an outlet passage at the upper end. With each case,there is provided at least one partition of generally frustonicalconfiguration and preferably a succession of such partitions. Forexample, in FIG. 5, a single frustocone F is provided with a convergencyin the direction of flow of the fluid. The cone consists of a pair offrustoconical walls 504 which are apertured at the coolant and a packing503 of the coated-particle nuclear fuel. A plate can be provided at 504to close the end of the cone. In the system of FIG. 6, the frustocones Fand F are provided with their apexes in axial abutment and their baseshuggingwthe wall of thecasing 501a. Each cone comprises a pair ofperforated walls 504a receiving the coated particles 503a between them.A similar structure is provided in FIG. 7 wherein the frustocones F andF have arcuate generatrices and are defined by a tube of generallysinusoidal axial cross section with a constriction at the center C InFIG. 13, we show an arrangement wherein the particles are received inperforated tubes 404 and are separated as zones 412 by layers, mats orwebs of graphitized fibers 411.

We claim:

1. A fuel element for a nuclear reactor comprising a casing defining aninlet duct, an outlet duct and a passage between said ducts; and acoolant-permeable partition spanning said passage, said partitionincluding a pair of fluid-permeable support walls defining a space ofsubstantially constant thickness between them, and a fluid-permeablelayer of substantially constant thickness of coated-particle nuclearfuel between said walls and filling said space, said partition beinggenerally rotationally symmetrical about an axis and converging towardsaid axis in the direction of flow of a coolant therethrough, theporosity of said walls and said layer being dimensioned to maintain asubstantially constant velocity of the coolant in the axial direction,said partition diverging in the direction opposite said direction offlow and terminating at said casing.

2. The fuel element defined in claim 1 wherein said partition isgenerally conical.

3. The fuel element defined in claim 2 wherein said partition consistsof a plurality of cones having apexes in axial alignment and insubstantial contact.

4. The fuel element defined in claim 1 wherein said partition has theconfiguration of a corrugated tube.

5. The fuel element defined in claim 1 wherein the diameter of thecoated particles in one portion of the element differs from the diameterof coated particles in another portion thereof.

6. The fuel element defined in claim 1 wherein the nuclear-fuel contentof the particles in one portion of the'elei'nen't differs from thenuclear-fuel content of the particles in another portion thereof.

7. The fuel element defined in claim 1 wherein said walls aresubstantially rigid, further comprising at least one layer of arefractory fluid-permeable flexible material of low-neutron absorptioncross section in contact with said layer of said particles for absorbingexpansion and contraction thereof.

8. The fuel element defined in claim 7 wherein said layer of low neutronabsorption cross section is dis posed within said layer of particles.

9. The fuel element defined in claim 6 wherein said layer of low neutronabsorption cross section is disposed between said layer of particles andone of said walls.

10. The fuel element defined in claim 1 wherein the thickness and/orporosity of said layer and/or the thickness and/or the porosity of saidwalls is varied over the length of the fuel element for controlleddistribution of a coolant therealone.

11. The fuel element defined in claim 1 wherein said layer is a mat feltor fabric of graphite, silica or like fibers capable or resisting thehigh temperatures of the reactor and yielding to the thermally inducedexpansion and contraction of the nuclear fuel.

1. A fuel element for a nuclear reactor comprising a casing defining aninlet duct, an outlet duct and a passage between said ducts; and acoolant-permeable partition spanning said passage, said partitionincluding a pair of fluid-permeable support walls defining a space ofsubstantially constant thickness between them, and a fluid-permeablelayer of substantially constant thickness of coated-particle nuclearfuel between said walls and filling said space, said partition beinggenerally rotationally symmetrical about an axis and converging towardsaid axis in the direction of flow of a coolant therethrough, theporosity of said walls and said layer being dimensioned to maintain asubstantially constant velocity of the coolant in the axial direction,said partition diverging in the direction opposite said direction offlow and terminating at said casing.
 2. The fuel element defined inclaim 1 wherein said partition is generally conical.
 3. The fuel elementdefined in claim 2 wherein said partition consists of a plurality ofcones having apexes in axial alignment and in substantial contact. 4.The fuel element defined in claim 1 wherein said partition has theconfiguration of a corrugated tube.
 5. The fuel element defined in claim1 wherein the diameter of the coated particles in one portion of theelement differs from the diameter of coated particles in another portionthereof.
 6. The fuel element defined in claim 1 wherein the nuclear-fuelcontent of the particles in one portion of the element differs from thenuclear-fuel content of the particles in another portion thereof.
 7. Thefuel element defined in claim 1 wherein said walls are substantiallyrigid, further comprising at least one layer of a refractoryfluid-permeable flexible material of low-neutron absorption crosssection in contact with said layer of said particles for absorbingexpansion and contraction thereof.
 8. The fuel element defined in claim7 wherein said layer of low neutron absorption cross section is disposedwithin said layer of particles.
 9. The fuel element defined in claim 6wherein said layer of low neutron absorption cross section is disposedbetween said layer of particles and one of said walls.
 10. The fuelelement defined in claim 1 wherein the thickness and/or porosity of saidlayer and/or the thickness and/or the porosity of said walls is variedover the length of the fuel element for controlled distribution of acoolant therealone.
 11. The fuel element defined in claim 1 wherein saidlayer is a mat felt or fabric of graphite, silica or like fibers capableor resisting the high temperatures of the reactor and yielding to thethermally induced expansion and contraction of the nuclear fuel.